1. Field of the Invention
The invention generally relates to a vaccine for treating or preventing the establishment of latent tuberculosis infections. In particular, the invention provides a recombinant mycobacterium that overexpresses the transcription factor DosR at a level sufficient to cause induction of the dosR regulon even under non-inducing conditions.
2. Background of the Invention
Tuberculosis (TB) is a global public health problem resulting in 8 million new cases and 2 million deaths each year. An estimated 2 billion persons (one-third of the world's population) are latently infected with Mycobacterium tuberculosis (Mtb) Reactivation of latent tuberculosis accounts for most new cases of active disease. Reactivation from latent TB (i.e. active tuberculosis) is associated with inflammation, necrosis and cavitation of the lung, a process that results in draining of the lesions into the bronchus. The resulting cough-generated aerosol causes dissemination of the organism to uninfected, susceptible persons, thus maintaining a transmission chain.
The ability of Mtb to enter a latent state has been investigated. Cole (Nature 393:537-544, 1998) identified a putative phosphorylation-dependant transcription factor Rv3133c that is upregulated immediately upon entry into dormancy. Boon and Dick (J. Bacteriol. 184: 6760-6767, 2002) demonstrated that disruption of dormancy-induced Rv3133c resulted in the loss of the ability of BCG to enter and maintain the dormant state, and that upregulation of several dormancy proteins required the Rv3133c response regulator. Based on these two functions, dormancy survival and regulation, they named the Rv3133c gene dosR for “dormancy survival regulator”. The DosR protein is thus a key regulator in the mycobacterial dormancy response. Further investigations (e.g. Voskuil et al., J. Exp Med. 198:705-713, 2003) demonstrated that the induction of a set of 48 genes (including dosR) adapts the Mtb organism for survival during extended periods of dormancy. The 48-gene set is known as the dormancy regulon or dosR regulon.
Current vaccine strategies are focused on the use of the Bacillus Calmette-Guérin (BCG) vaccine for the immunization of healthy persons who have never been in contact with M. tuberculosis. This strategy has been of questionable efficacy and only addresses stopping active or progressive disease, without taking into account the relevance of the latent, carrier population, particularly in countries where tuberculosis is endemic. Further, the efficacy of BCG is disputed, with various controlled clinical trials and case control studies showing estimates of protection ranging from zero to 83%. A metaanalysis of the results of these trials led one group to conclude that the level of protection against pulmonary tuberculosis was 50% if the effect of latitude was included in a regression model, whereas another group concluded that these data were too heterogeneous for meaningful analysis. Both the meta-analyses concluded that there was significant protection against disseminated forms of disease, such as tuberculous meningitis and miliary tuberculosis, with efficacy estimates ranging between 64% and 86%, but little protection was afforded by BCG vaccination against the development of pulmonary tuberculosis in adolescents or adults (Mahommed et al., Pediatr Infect Dis J., 25(12): 1167-72, 2006). Thus, the efficacy of the current BCG vaccine is debatable.
Most current strategies for the development of new TB vaccines are also focused on preventing active or progressive disease by using attenuated variants of M. tuberculosis. Examples include auxotrophs, e.g. for amino acids (Lee et al., Infect Immun., 74(11):6491-6495, 2006; Sampson et al., Infect Immun. 72(5):3031-7, 2004), for nucleotide metabolism (Brown et al, Infect Immun. 2005 January; 73(1):666-70, 2005; Sambandamurthy et al., Nat. Med. 8(10):1171-4, 2002), devoid of transcriptional regulators like PhoP (Martin et al., Vaccine, 24(17):3408-19, 2006) or unable to produce some lipids, like the FadD26 mutant (Infante et al., Clin Exp Immunol., 141(1):21-8, 2005). However, vaccines based on such strategies still do not address the relevance of the persistent stage of tuberculosis.
International patent application publication number WO 2006/104389 (Leiden University Medical Center) does address the relevance of the persistent stage of tuberculosis. However, this application discloses only subsets of dosR regulon latency-associated proteins that are purportedly useful for eliciting an immune response. The application is focused entirely on provision of the proteins as antigens, and DosR is among the antigens that are listed. However, DosR is not a preferred antigen, nor is induction of genes in the dosR regulon suggested or considered.
The prior art has thus far failed to provide a tuberculosis vaccine that provides protection against the development, maintenance and/or reactivation of latent tuberculosis infection, and such a vaccine would be highly beneficial. For example, because reactivation is typically associated with some type of immunosuppressing condition (e.g. diabetes, aging, etc.) that allows a latent lesion to progress to active disease, it would be highly beneficial to have available a vaccine that thwarts this progression, e.g. by promoting elimination of mycobacteria that are in the latent state. No such vaccine is currently available. Further, no current vaccine, and no vaccine under development, is designed to take advantage of the properties of the DosR transcriptional regulator.